Paget's disease is a long-lasting (chronic) disorder that causes abnormal bone growth. Paget's disease most often affects the bones in the pelvis, spine, skull, chest, and legs.

In healthy people, bone is constantly being replaced as bone tissue is broken down and absorbed into the body, then rebuilt with new cells. In the early stages of Paget's disease, bone tissue breaks down faster than it rebuilds. To make up for this breakdown process, the body speeds up the rebuilding process. However, this new bone is often weak and brittle, causing it to break (fracture) more easily.

Most cases of Paget's disease do not cause symptoms. However, the most common symptoms, when they occur, are bone pain and deformed bones (bowed legs, enlarged skull or hips, or a curved backbone).

Paget's disease is most common in middle-aged and older adults. It may be treated with medicines or, in rare cases, with surgery.